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Low Pressure Ratio Cascaded Joule-Thomson Cryogenic Coolers

Keywords: Micro Cryogenic Cooler, Joule-Thomson, Low Pressure Ratio

This work presents the design,
fabrication, and characterization of
two micro/mini coldstages intended to
investigate the feasibility of low
pressure ratio cascaded Joule-Thomson
(JT) based highly miniaturized
cryogenic coolers. Single-stage
single-refrigerant JT coldstages
require pressures ratios greater than
8:0.1 MPa to reach cryogenic
temperatures (Lerou et al., 2006). Low
pressure ratio single-refrigerant
coldstages have a small temperature
range of cooling, limiting the
ultimate temperature a single stage
can reach (Ray Radebaugh, 2014).
Coldstages designed to work with
custom mixed refrigerants to reach
cryogenic temperatures can be designed
to operate with lower pressure ratios
of less than 0.4:0.1 MPa (Wang, Lewis,
Lin, Radebaugh, & Lee, 2013). However,
mixed refrigerants have an order of
magnitude lower specific refrigeration
capacity when compared to single
component refrigerants and can be
prone to stratification of the
refrigerant, further reducing
refrigeration capacity (Lewis, Wang,
Schneider, Lee, & Radebaugh, 2013).
The gas-phase refrigerants require
compression, and the pressure ratio
and flow rate of micro JT coolers are
currently limited by the capability of
modern mini/micro-compressors (R
Radebaugh, Bradley, Coolidge, Lewis, &
Lee, 2014). Thus, micro JT coolers
have been operated by compressors that
are much larger in size, or from high-
pressure gas cylinders, thus limiting
overall system miniaturization. A
cascaded coldstage presents a
practical solution to this. The
cascaded system consists several self-
contained low pressure ratio single
refrigerant JT cycles. Each stage has
a small temperature range of cooling
and is used to precool subsequent
stages. Since the pressure ratios are
lower, single-refrigerant JT coolers
are compatible with mini/micro-
compressors, thus enabling the use of
such compressors to achieve overall
system miniaturization. Due to the
limited temperature range of each
single-refrigerant cooling stage,
several stages can be cascaded
together to reach cryogenic
temperatures. This work presents two
cascaded coldstages with JT
restrictions fabricated from glass
capillaries with inner diameters of 50
microns and inter stage polyimide tube
in tube heat exchangers with annular
channels of 75 microns. The coldstages
investigated use single component
refrigerants operating at pressure
ratios of less than 0.5:0.1 MPa. The
JT restriction capillary dimensions
for the coldstages developed in this
work are determined by laminar flow
undergoing adiabatic expansion through
a circular tube. The pressure
dependent properties of the two phase
flow are evaluated over the length of
the restriction via tabular
integration for each refrigerant using
properties from NIST REFPROP v. 9.1
(National Institute of Standards and
Technology, Gaithersberg, 2016). A two
stage coldstage is shown to cool from
293K to 228 K with a net refrigeration
power of 150 mW. A three stage
coldstage is shown to cool from 293 K
to 193 K with net refrigeration power
of 15 mW. The refrigerants for the
first, second and third stages were
isobutane, propane, and R-116. The
coldstages presented demonstrate the
feasibility of low pressure ratio
cascaded JT cryogenic coolers.